Various prior art articles and papers discuss drill pipe design considerations. Those familiar with the design, manufacture and use of drill pipe recognize that a selected drill pipe design is based upon numerous factors, including casing/hole considerations, predicted well loads, and hydraulic requirements. The manufacture of drill pipe also takes into consideration metallurgy, high-torque tool joints, and top drive matching strength.
Although the existing drill pipe is available in various standard sizes, it frequently does not meet all the objectives of the driller. The deficiencies of existing drill pipe are particularly acute when the driller is engaged in extended reach drilling (ERD). A standard 5-inch drill pipe is generally considered inadequate for most ERD applications due to hydraulic and torsional limitations. In most applications, ERD projects utilize either or both 51/2-inch and 65/8-inch drill pipe, which is commercially available in standard sizes from various companies.
One of the significant deficiencies of 51/2-inch drill pipe is its hydraulic limitations, particularly when drilling deep wells. An exemplary 51/2-inch drill pipe string provides the ability to sustain 787 kips in tension and from 33 ft-kips to 45 ft-kips or more in torsion, depending on the tool joint selection. While a 51/2-inch drill pipe string is generally able to sustain the desired drilling torque and tensile loads, this size drill string commonly provides a very high pressure drop at conventional flow rates. Commonly from 900 to 1100 gpm is required to effectively clean a 121/4 inch hole at high inclinations, and practical experience has shown that undesirable pressure losses are experienced when drilling under these conditions. A 51/2-inch drill pipe is thus inadequate for many ERD applications and has substantial hydraulic limitations in other applications, particularly when used in long hole sections.
An alternative available to the driller is to utilize the next commercially available size of drill pipe, namely 65/8-inch drill pipe, to reduce hydraulic pressure losses at the elevated flow rates required for effective hole cleaning. The problem is that a 65/8-inch drill pipe represents an over-design from the viewpoint of drill loads, is inefficient from a space standpoint, and can limit setback capability. Moreover, 65/8-inch drill pipe may require rig modifications for implementation compared to using 51/2-inch drill pipe, and 65/8-inch drill pipe cannot be used inside 95/8-inch casing or 81/2-inch open hole. The 65/8-inch drill pipe thus overcomes most of the hydraulic limitations of a 51/2 drill pipe, but is over-designed structurally, is dimensionally inefficient, may not be used after 95/8-inch casing is set, and requires a variety of associated upgrades.
The disadvantages of the prior art are overcome by the present invention, and an improved drill pipe is hereinafter disclosed that has enhanced strength and fluid flow characteristics and in many applications will not have the disadvantages of either 51/2-inch or 65/8-inch drill pipe.